Heretofore, in a radio communication system that uses the WCDMA (Wideband Code Division Multiple Access) method, progress has been made in standardizing a technology that performs packet extension called HSDPA (High Speed Downlink Packet Access). In this radio communication system, communication is performed on conventional WCDMA channels called DPCH (Dedicated downlink Physical channels) while outbound reception of a high-speed channel called an HSDSCH channel (High Speed Downlink Shared Channel) is simultaneously performed in parallel.
In this HSDSCH channel, a technology called adaptive modulation is used whereby the modulation method and coding rate are changed in accordance with propagation path conditions, and each mobile station apparatus notifies a base station apparatus of propagation path conditions by sending an outbound pilot channel reception condition report called a CQI (Channel Quality Indicator) report.
FIG. 1 is a drawing for explaining CQI values that indicate propagation path quality. An example is illustrated in which 22 levels of CQI value are used, corresponding to CQI values 0 through 21. In this case, a mobile station apparatus reports a low CQI value to a base station apparatus when it determines that propagation path conditions are poor, and reports a high CQI value to a base station apparatus when it determines that propagation path conditions are good. A CQI value of 0 is reported by a mobile station apparatus when the base signal reception level does not reach a predetermined threshold value (i.e. when out of range).
A base station apparatus determines a mobile station apparatus to be made a transmission target based on the CQI values reported by each mobile station apparatus, and sends control information such as the modulation method, number of multiplex codes, and number of information bits, to that mobile station apparatus by means of HS-SCCH (High Speed Shared Control Channel). A mobile station apparatus receives the HS-SCCH and determines whether or not there is transmission addressed to itself, and if it determines that transmission is addressed to itself, demodulates and decodes data using information conveyed in the control information.
An example of a conventional radio communication system that uses this kind of adaptive modulation method in a code division multiple access system is a method whereby data for transmission is duplicated for a mobile station apparatus in the vicinity of a cell boundary, and duplicated identical data is transmitted spread with a plurality of codes and multiplexed, as disclosed in Unexamined Japanese Patent Publication No. 2002-232943, for example.
However, in the conventional radio communication system disclosed in Unexamined Japanese Patent Publication No. 2002-232943, there are problems in that, if mobile terminal apparatuses supporting HSDPA are widely distributed in the area covered by a base station apparatus when the base station apparatus compensates for minimum throughput for all mobile station apparatuses using HSDPA, the time for assigning HSDPA to a mobile station apparatus far from the base station apparatus becomes lengthy, and as a result, overall system throughput falls, and also, even though the HSDPA time period assigned to a mobile station apparatus in the vicinity of a cell edge is small, that mobile station apparatus must send a CQI value periodically, and therefore, since that mobile station apparatus is far from the base station apparatus, there is a significant waste of power.
On the other hand, a problem with the above-described conventional radio communication system is that, since the HS-SCCH, the physical channel that carries control signals including the modulation method, information bit length, and number of multiplex codes to a mobile station apparatus that is the object of data transmission is a transmission from a single base station apparatus, as with HSDPA, there is no soft handover, and therefore extremely high power must be used for transmission from a base station apparatus to a distant mobile station apparatus at the cell edge, so that, when a base station apparatus frequently assigns HSDPA to a mobile station apparatus at the cell edge, the proportion of total transmission power accounted for by control channel power is high, and consequently overall system throughput is poor.